The gasification of various fossil fuels, especially solid carbonaceous fuels such as coal, has received much attention in recent years as an alternative to the direct combustion of fossil fuels. In these systems, fuel and oxidant are reacted substoichiometrically in a vessel to produce a product fuel gas containing carbon monoxide, CO, and hydrogen, H.sub.2. This fuel gas, after the removal of unreacted or inert particulate and any sulfur bearing compounds which may also have been produced in the gasification reaction, can be used in a variety of processess which require a clean burning fuel.
One particular use of gasification technology is projected in the electric generating industry. A gasifier supplying clean product gas to a combined cycle electric generation plant may achieve overall energy conversion efficiencies up to ten percentage points higher than conventional electric power generating plants employing high pressure steam in a Rankine cycle.
In an advanced combined cycle generation plant a solid fuel, such as coal, would be gasified with an oxidant, such as air or oxygen, to form a product gas rich in CO and H.sub.2. This product gas would be cleaned of possible entrained particulates and sulfur compounds and used as fuel for the combustor of a gas turbine generator set. The high temperature exhaust from the gas turbine outlet, composed of the products of combustion of the product fuel gas and air, would then be routed to a heat recovery boiler for the generation of high pressure steam. The steam thus generated, in addition to any steam which may have been raised in the gasification or gas cleanup processes, would be used in a conventional steam turbine cycle to generate additional electric power.
It is desirable for large electric generating plants in general to operate by means of automatic control systems which seek to optimize plant performance and plant safety. A gasification combined cycle electric generating plant as described above would preferably have such automatic controls to regulate the flow of input materials and output power. In particular, modern electric utilities rely on remote dispatch of a number of generating facilities whereby the desired output of each generating plant in the utility grid is determined and the information sent to each plant operator. Depending on consumer demand and economic factors, the desired load for a particular plant may vary throughout the operating day, requiring plant output to respond in an accurate and timely fashion.
In addition to the changing load demands placed upon a plant by the utility dispatcher, the automatic control system must also compensate for variations in plant performance caused by expected but unpredictable factors. These variations may be induced by a change in coal heating value, which may vary up to ten percent from a given value depending on what section of the coal mine or coal pile it was taken, changes in equipment performance caused by deterioration, maintenance, fouling, etc., or any other variation of individual equipment output which may effect overall plant performance.
Modern electric generating stations use automatic controls of the feedback type wherein plant output is measured and used to modify feed inputs in order to maintain desired electrical output. In a conventional steam electric generating plant the actual electric output is compared against the electric output desired by the dispatcher and fuel feed rate into the combustion furnace is regulated to match the actual and desired outputs. Air flow to the combustion furnace is regulated based on the amount of O.sub.2 present in the combustion products from the furnace, typically in the range of two to ten percent by volume. As long as air flow is maintained at a rate greater than the stoichiometric requirement of the fuel currently being fed, the amount of energy released within the combustion furnace will be equal to the chemical energy present in the fuel. Since the energy released within the combustion furnace is directly related to the steam energy produced by the furnace and associated heat recovery equipment, the regulation of fuel feed directly affects the electric power output, thus making the system responsive and well behaved in a control sense.
In the case of a coal gasifier combined cycle electric generating plant, however, this method of control does not provide the same advantageous results due to a number of fundamental differences between the two systems. First, the coal gasification combined cycle plant does not have products of combustion until after the combustion of the product gas in the gas turbine combustor or, alternatively, in a combustion furnace downstream of the product gas cleanup equipment.
The rate of oxidant feed to the gasifier is a factor in the product gas heating value and must be accurately regulated for overall cycle efficiency and safety. Should the fuel to oxidant feed ratio fall too low, a poor quality gas will be produced and excessive chemical energy released within the gasifier, resulting in inefficient generation of power and overloading of the gas cleanup equipment. A deficiency of oxidant within the gasifier will likewise cause inefficiency in the form of incomplete gasification and/or the production of unwanted hydrocarbons and unreacted carbon particules. A typical fuel to oxidant ratio is 0.2 for coal fuel and air oxidant.
The use of plant output as a basis for regulating fuel feed to the gasifier is also unsuitable due to the particular nature of the gasification reactions which take place therein. Since in a gasifier the fuel is being reacted under substoichiometric conditions, an increase in fuel feed without a concurrent increase in oxidant flow will not immediately produce an increased energy release within the gasifier. In fact, owing to the particular nature of substiochiometric combustion, the electric generating plant will actually experience an immediate decrease of power output following a unilateral increase in fuel feed to the gasifier.
This decrease results from the inverse relationship of coal feed to reaction temperature under substoichiometric conditions, thus causing a decrease in heat absorption in the downstream product gas heat absorption equipment. The increased gas heating value resulting from the increased coal to oxidant feed ratio will eventually reach the downstream gas turbine or combustion furnace thus resulting in increased plant output. With present designs of gasification combined cycle generating plants, however, this delay could be up to ninety seconds or longer. A conventional control system which attempts to regulate gasifier fuel feed based on plant electrical output would therefore require a large amount of damping or delay in order to remain stable.
Currently operating gasifier systems are of the demonstration or test variety and have not been required to function under utility generating plant conditions. In particular, for test gasifier systems, fuel and oxidant feed rates must be varied over a wide range for the purposes of evaluating the gasification process. This is usually accomplished by measuring the inputs to the gasifier by means of gravimetric feeders for the fuel and calibrated orfices for the oxidant and then measuring output gas flow rate and heating value to arrive at an overall system heat balance. With the accurate instrumentation and the concentration of technical personnel generally present on such test or demonstration gasification systems, it is unnecessary to rely on feedback control systems to regulate gasifier operation.
In a large electric generating system employing a gasification process this method would be unsuitable both from a manpower and from a control standpoint. Electric generating plants are subject to one overriding demand, the production of the proper amount of electric power for the generating grid. It is true that the feed rates of fuel and air have an effect on the efficiency with which the fuel is converted into electric power, but this is of secondary importance with regard to the need for the plant to quickly and accurately respond to the utility demand for power.
What is needed is a simple, effective system for regulating the flow of fuel and oxidant into a gasifier which is particularly suited to the unique nature of the gasification process and which is compatible with the realities of utility electric power generation.